drm/amdgpu/display: drop unused function

This is not longer used as of the latest rework of this
code so drop it to avoid a unused function warning.
Acked-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -2827,24 +2827,6 @@ dm_atomic_get_new_state(struct drm_atomic_state *state)
 	return NULL;
 }
 
-static struct dm_atomic_state *
-dm_atomic_get_old_state(struct drm_atomic_state *state)
-{
-	struct drm_device *dev = state->dev;
-	struct amdgpu_device *adev = dev->dev_private;
-	struct amdgpu_display_manager *dm = &adev->dm;
-	struct drm_private_obj *obj;
-	struct drm_private_state *old_obj_state;
-	int i;
-
-	for_each_old_private_obj_in_state(state, obj, old_obj_state, i) {
-		if (obj->funcs == dm->atomic_obj.funcs)
-			return to_dm_atomic_state(old_obj_state);
-	}
-
-	return NULL;
-}
-
 static struct drm_private_state *
 dm_atomic_duplicate_state(struct drm_private_obj *obj)
 {